Study Examines Prevalence of Schizophrenia Cases Associated with Cannabis Use Disorder After Cannabis Legalization

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In a recently published study, researchers examined the potential connections between cannabis legalization in Ontario, Canada and the prevalence of schizophrenia diagnoses. The study was conducted from January 1, 2006, to December 31, 2022, which covers both the legalization of medical cannabis, which started in 2001 and expanded until full legalization in 2015, and nonmedical cannabis legalization in 2018. The population-based cohort study, “Changes in Incident Schizophrenia Diagnoses Associated With Cannabis Use Disorder After Cannabis Legalization,” was published in February 2025 in JAMA Network Open (1). It was funded by Canadian Institutes of Health Research and by ICES (formerly known as the Institute for Clinical Evaluative Sciences).

The study noted prior research on associations of cannabis use, especially cannabis with high THC content, and the development or earlier onset of psychosis and schizophrenia, though evidence on cannabis legalization and psychosis is deficient. The goal in this study was to measure the population-attributable risk fraction (PARF) for cannabis use disorder (CUD) associated with schizophrenia, in other words, the number of cases of schizophrenia that could have been prevented if cannabis use severe enough to require emergency or hospital care was eliminated.

This study analyzed data from 13,588,681 people aged 14 to 65 years without a history of schizophrenia, noting diagnoses of CUD diagnoses in emergency rooms or hospital settings.

Some of the findings included:

• Out of the total population, 118,650 individuals had CUD
• 91,106 developed schizophrenia (0.6% of the population without CUD and 8.9% of the population with CUD)
• From the prelegalization period to the legalization period, the PARF for CUD associated with schizophrenia increased from 3.7% to 10.3%
• Compared with the PARF for CUD associated with schizophrenia, which increased steadily even after cannabis policy changes, the PARF for CUD associated with psychosis not otherwise specified accelerated after the liberalization of medical cannabis policy

Limitations of the study were noted. These included a lack of access to confounders (including individual-level income, educational attainment, family history of mental health disorders, and genetics), which could affect the validity of the PARF estimators. Other limitations included the exposure definition of CUD and the impacts of COVID-19.

Ultimately, the researchers concluded: “the proportion of incident cases of schizophrenia associated with CUD almost tripled during a period encompassing ongoing liberalization of medical and nonmedical cannabis. Although the proportion of cases of schizophrenia associated with CUD increased fairly linearly over time, incident cases of psychosis NOS and the proportion associated with CUD accelerated after cannabis liberalization.”

More research was called for in order to understand cannabis legalization policy’s long-term associations with the occurrence of psychotic disorders. Additionally, it was suggested that the association between cannabis use disorders and schizophrenia be taken into consideration in the legalization of cannabis.

Reference

1. Myran, DT.; Pugliese, M.; Harrison, LD.; Solmi, M.; Anderson, KK.; Fiedorowicz, JG.; Finkelstein, Y.; Manuel, D.; Taljaard, M.; Webber, C.; Tanuseputro, P. Changes in Incident Schizophrenia Diagnoses Associated With Cannabis Use Disorder After Cannabis Legalization. JAMA Netw Open. 2025. DOI: 10.1001/jamanetworkopen.2024.57868